1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted of thin film transistors (hereinafter referred to as TFTs) and a method of manufacturing the same. More specifically, the present invention relates to an electro-optical device typified by a liquid crystal display panel and electronic equipment that mounts such an electro-optical device as its component.
It is to be noted that semiconductor devices through this specification refer to devices that can function by utilizing semiconductor characteristics in general, and electro-optical devices, semiconductor circuits, and electronic equipment are all the semiconductor devices.
2. Description of the Related Art
In recent years, development of a semiconductor device having an integrated circuit with large area constituted of thin film transistors (TFTs) formed of a semiconductor thin film (approximately several to several hundred nm in thickness) formed on a substrate having an insulating surface is making progress. An active matrix liquid crystal display device, an EL display device, and a contact type image sensor are known as typical examples of the semiconductor device. In particular, since a TFT in which a crystalline silicon film (typically, a poly-silicon film) is an active layer (hereafter referred to as a poly-silicon TFT) has high field-effect mobility, the TFT can constitute various functional circuits.
For example, in an active matrix liquid crystal display device, a driver circuit for controlling a pixel circuit for displaying images with each functional block and pixel circuits such as a shift register circuit with a CMOS circuit as a base, a level shifter circuit, a buffer circuit, and a sampling circuit is formed on a substrate.
TFTs (pixel TFTs) are arranged in several tens to several million pixels, respectively, in the pixel circuit of the active matrix liquid crystal display device, and the pixel TFTs are provided with pixel electrodes, respectively. An opposing electrode is provided on the side of an opposing substrate sandwiching liquid crystal with the substrate to thereby form a condenser with the liquid crystal as dielectric. The voltage applied to the respective pixels is controlled by a switching function of the TFT to drive the liquid crystal by controlling electric charge to the condenser. Thus, transmission light amount is controlled to display images.
The pixel TFT consists of an n-channel TFT and is driven as a switching element by applying a voltage to the liquid crystal. Since the liquid crystal is driven with an alternating current, a method called frame inversion driving is adopted in many cases. In this method, it is important to sufficiently lower an off current value (a drain current that flows during off operation of a TFT) as a characteristic required to the pixel TFT in order to suppress power consumption.
A lightly doped drain (LDD) structure is known as a TFT structure for reducing an off current value. This structure is such that a region added with an impurity element at low concentration is provided between a channel forming region and a source region or a drain region formed by adding an impurity element at high concentration, and the provided region is called an LDD region. Further, a GOLD (gate-drain overlapped LDD) structure in which an LDD region and a gate electrode are overlapped with each other through a gate insulating film is known as means for preventing deterioration in an on current value due to hot carrier. It is known that such a structure makes a high electric field in the vicinity of a drain to be relaxed to prevent hot carrier injection, and thus, which is effective in preventing deterioration.
Although the GOLD structure is effective in preventing deterioration in an on current value, on the other hand, there has been a problem in that an off current value becomes large in the GOLD structure compared with a general LDD structure. Therefore, the GOLD structure is not preferable for being applied to a pixel TFT. On the contrary, the general LDD structure is effective in suppressing the off current value, but has a little effect in preventing deterioration due to hot carrier injection by relaxing an electric field in the vicinity of a drain. As described above, in a semiconductor device having a plurality of integrated circuits such as an active matrix liquid crystal display device, the above problem clearly exists as in particular, in a crystalline silicon TFT and is becoming more evident as performance required for the active matrix liquid crystal display device is enhanced.
Conventionally, when the TFT provided with the LDD structure or the TFT provided with the GOLD structure is to be formed, there is a problem in that the manufacturing process becomes complicated and the number of steps increases. It is clear that the increase in the number of steps becomes not only the cause of the increase in manufacturing cost but also the cause of the reduction in manufacturing yield.